I think the error message for a different condition was inadvertently
copied.

This problem seems to have been introduced by commit a4d75c8.

Author: Álvaro Herrera <alvherre@kurilemu.de>
Reported-by: jian he <jian.universality@gmail.com>
Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Backpatch-through: 14
Discussion: https://postgr.es/m/CACJufxEZ48toGH0Em_6vdsT57Y3L8pLF=DZCQ_gCii6=C3MeXw@mail.gmail.com

It's possible that if the only live partition is concurrently dropped
and try_table_open() fails, that the bms_del_member() will pfree the
live_parts Bitmapset.  Since the bms_del_member() call does not assign
the result back to the live_parts local variable, the while loop could
segfault as that variable would still reference the pfree'd Bitmapset.

Backpatch to 15. 52f3de8 was backpatched to 14, but there's no
bms_del_member() there due to live_parts not yet existing in RelOptInfo in
that version.  Technically there's no bug in version 15 as
bms_del_member() didn't pfree when the set became empty prior to
00b4146 (from v16).  Applied to v15 anyway to keep the code similar and
to avoid the bad coding pattern.

Author: Bernd Reiß <bd_reiss@gmx.at>
Reviewed-by: David Rowley <dgrowleyml@gmail.com>
Discussion: https://postgr.es/m/6b88f27a-c45c-4826-8e37-d61a04d90182@gmx.at
Backpatch-through: 15